Metal based contacts of semiconductors, such as in photovoltaic devices, involves the formation of electrically conductive contacts on the front side or the side of the semiconductor which is illuminated by incident light and the back side or the side which is not illuminated by incident light. The metal coating establishes Ohmic contact with the semiconductor so that charge carriers emerge from the semiconductor into the electrically conductive contacts without losses and with long lifetimes. In order to avoid current loss, metalized contact grids have adequate current conductivities, e.g., a high conductivity or a sufficiently high conductor track cross section.
Numerous processes which meet the above requirements exist for metal coating the back side of solar cells. For example, in order to improve current conduction at the back side of solar cells, p-doping directly under the back side is increased. Usually aluminum is used for this purpose. The aluminum is applied, for example, by vapor deposition or by being printed onto the back side and being driven in or, respectively, alloyed in. When metal coating the front sides or illuminated sides, the objective is to achieve the least amount of shading of the active semiconductor surface in order to use as much of the surface as possible for capturing photons.
The method used commercially to form the front side contacts in solar cells is the application of a metallic paste by screen printing. The paste contains metal particles (typically silver) to provide electrical conductivity, as well as glass frit, rheology modifiers, and a high boiling solvent, such as terpineol. After printing, the cell is dried, and then typically fired in a belt furnace at temperatures ranging from about 600-1000° C. Upon firing, the glass frit will react with, or “burn-through”, an antireflection coating (typically silicon nitride) on the front side and helps provide adhesion to the cell. Use of a screen printed paste is the industry standard but has disadvantages. Screen printing is a contact method of printing, requiring significant handling of fragile silicon solar cells, resulting in a significant amount of accidental breakage. It also generates chemical waste, municipal waste, and an added expense in the form of broken screens. Finally, the smallest line widths that can be generated in production are physically limited by screen technology to be in the range of about 80-100 microns. Smaller line widths by screen printing may be physically possible in the laboratory, but are more difficult to achieve in mass production at the present time.
More complex processes for producing the front side contacts make use of laser or photolithographic techniques for the definition of current track structures. Currently these techniques can produce narrower lines but at the expense of through-put. The current tracks are then metalized. In general, various metal coating steps are often used in order to apply the metal coating in attempting to achieve sufficient adhesive strength and a desired thickness for electrical conductivity. For example, when wet-chemical metal coating processes are used, a first fine metal coating is deposited on the current tracks by means of palladium catalyst. This is often reinforced with electroless deposition of nickel. In order to increase the conductivity, copper may be deposited on the nickel by electroless or electrolytic deposition. The copper may then be coated with a fine layer of tin or silver to protect it from oxidation.
Alternatively, current tracks may be metalized using a light induced plating process. This metallization process involves first metalizing the back side of the solar cell using conventional methods of printing and sintering an electrically conductive paste in an inert gas atmosphere. Such pastes may include silver, aluminum and frit with organic binders. Other metals such as nickel, palladium, copper, zinc and tin also may be burned in the paste. The front side of the solar cell is coated with a passivation or antireflection layer of silicon oxide or silicon nitride. Trenches for current tracks are formed in the antireflection layer which extends into the semiconductor. Trenches may be formed using photolithography, laser writing or mechanical erosion. The current tracks of the front side are then plated with nickel by light induced plating. The solar cell is placed in a nickel plating bath and light is applied to the solar cell and a nickel layer is generated on the semiconductor material after approximately 1-2 minutes. A further metal layer, such as copper, may be directly generated over this nickel layer for reinforcement. The copper layer may be protected from oxidation by applying a thin layer of silver or tin over the copper layer.
Another method of forming metal contacts on a solar cell is as follows. Current tracks are formed by using a laser which selectively removes portions of the antireflective layer to expose the underlying semiconductor material. However, laser applications are costly and in general less costly methods are preferred in the industry Ink which contains metal nanoparticles in the range of about 20 nm to 1000 nm is applied to the exposed semiconductor material by inkjet or aerosol apparatus. The device is heated to temperatures of about 100° C. to 900° C. for a term lasting from one second to thirty minutes to drive off any solvent and form the metal contacts. These contacts are then reinforced with additional metal layers by electroplating.
Although there are methods for forming metal contacts on semiconductors, there is still a need for an improved method of making the initial metal contacts on semiconductors.